In a vapor-compression type refrigerator using a refrigerant of carbon dioxide (CO2), a refrigerant pressure is needed to reach and exceed a critical pressure of the refrigerant in a high-pressure portion when an ambient temperature is high (more than 30 degrees Celsius [° C.]). The pressure at the high-pressure portion is thereby approximately ten times as high as that of a vapor-compression type refrigerator using a refrigerant of chlorofluorocarbon (CFC); accordingly, the pressure at the low-pressure portion is also approximately ten times as high as that of the vapor-compression type refrigerator using the refrigerant of chlorofluorocarbon.
Cross-sectional areas of refrigerant channels are therefore circular or elliptic so that withstanding pressure can be increased (refer to JP-A-2000-111290 [U.S. Pat. No. 6,357,522 B2]). However, in a viewpoint of heat conductivity, an angled cross-sectional area (e.g., rectangular) is preferable. This angled cross-sectional area is described in JP-A-2000-356488 (JP3313086 B2), which provides an optimum example of a heat exchanger at a supercritical pressure. However, since its usage pressure falls within a high-pressure region (about 10 MPa) of a CO2 cycle, it does not provide an optimum example as an evaporator. Further, it provides; without specification of used material, no optimum pressure-withstanding design for a CO2 cycle that is operated especially at high pressures. Further, a refrigerant state is different between an evaporator and heat exchanger, so that contribution of a shape should be considered with respect to a refrigerant-side performance.
Further, rectangular cross-sectional areas of refrigerant channels having arcuate corners in JP-A-2000-356488, are inferior in heat conductivity to those having angled (not-arcuate) corners. In comparison with the arcuate corners (e.g., circular corners) having equivalent cross-sectional areas, the angled corners secure broader conductive areas in the refrigerant side, and thicker annular liquid films, further enabling uneven distribution of the liquid. It is assumed that the foregoing phenomena remarkably contribute to nucleate boiling.
Thus, the heat exchangers described in JP-A-2000-356488 is suitable as a radiator at a high-pressure portion, not being directly applicable to heat an exchanger at a low-pressure portion such as an evaporator. Moreover, refrigerant channels having angled cross-sectional areas are potentially involved in tube damage owing to stress concentration. In particular, attention must be paid to the channels having corners of nearly right angles.